1. Field of the Invention
The present invention relates to a phase retarder.
2. Description of the Related Art
Phase retarders comprising a uniaxially oriented polymer film have been used as optical compensators to enhance display qualities of liquid crystal display devices.
A STN type liquid crystal display device using such phase retarders as compensators is advantageous in that it can be lighter in weight, thinner and less expensive, but disadvantageous in that it has narrow viewing angle characteristics and exhibits a low contrast ratio compared to a STN double cell type liquid crystal display device using a liquid crystal cell as optical compensators.
The STN type device has been considerably improved to alleviate its poor contrast by laminating two plies of phase retarders, but they are still unsatisfactory for the viewing angle characteristics.
The viewing angle characteristics of the liquid crystal display devices using the phase retarders as optical compensators are greatly depending upon the viewing angle characteristics of the phase retarders themselves. The viewing angle characteristics of the phase retarders comprising uniaxially oriented polymer films can be evaluated by using a viewing angle.
The viewing angle of phase retarders is defined as a tilt angle (.theta..sub.1.10) of a film relative to a normal direction when a ratio of the retardation (R) observed for the tilted film to the retardation (R.sub.0) observed for the normally disposed film is 1.10 with the film being tilted by rotating around an axis which is corresponding to the slow axis of the film having a positive birefringent anisotropy, or to the fast axis of the one having a negative birefringent anisotropy where the observation is performed under a polarizing microscope equipped with Senarmont compensator. The higher the viewing angle, the better the viewing angle characteristics.
As techniques for enhancing the viewing angle, there have been proposed several methods such as a method of shrinking the film in the direction perpendicular to the stretching direction at the time of stretching as disclosed in Japanese Patent KOKAI (Laid-open) No. 2-191904; a method of controlling the birefringent properties of a raw film, which has been produced from a molten polymer or a polymer solution under an electric field applied, by stretching as disclosed in Japanese Patent KOKAI (Laid-open) No. 2-285303; and a method of laminating a film, which has been produced under an electric field applied, on a conventional phase retarder as disclosed in Japanese Patent KOKAI (Laid-open) No. 3-85519. However, any one of those methods can hardly be said to be prominent in mass producibility.
Furthermore, in order to improve display characteristics such as contrast, color and viewing angle properties and the like, there have been developed various liquid crystal display schemes other than the STN type, for example, a TN liquid crystal display having a birefringent optical compensating layer as disclosed in G.B. Patent No. 1462978 and a homeotropic liquid crystal display scheme having a birefringence compensating layer as disclosed in Japanese Patent KOKAI (Laid-open) No. 2-15239. However, these liquid crystal displays require as optical compensator a phase retarder comprising a film having a non-anisotropic refractive index in the film plane (R.sub.0 =0) and a refractive index different therefrom in the thickness direction normal to the plane.